The present invention relates to a heating device using induction heating. In particular, this invention relates to a fixing device for fixing a toner image using a heating device in an electrophotographic copying apparatus or printer apparatus using a toner as a visible-image forming agent.
In a conventional fixing device incorporated in a copying apparatus using an electrophotographic process, a developer or a toner formed on an image-fixation medium is heated and fused and the toner is fixed on the image-fixation medium. Well-known toner heating methods applicable to the fixing device include a method using radiation heat obtained by turning on a filament lamp and a flash heating method using a flash lamp as a heat source.
In recent years, a fixing device using an induction heating device as a heat source has been proposed.
Jpn. Pat. Appln. KOKAI Publication No. 2-270293 discloses an induction heating device with two induction coils, which includes a plurality of inverter circuits and a detection circuit that detects a zero point of an AC power supply, wherein when switching elements of the inverter circuits are switched (i.e. driving of coil A is switched to driving of coil B), the switching is effected at the zero point of the AC power supply.
Jpn. Pat. Appln. KOKAI Publication No. 2000-206813 discloses a technique wherein when a heating roller (or a heating belt) is heated with use of a plurality of coils, the ratio of the amounts of power applied to the coils is varied so as to make uniform the heating temperature distribution in the longitudinal direction of the heating roller. It also discloses a technique wherein a difference between temperatures of temperature detection means is detected, and the ratio of the amounts of power applied to the respective coils is varied, thereby driving the coils at the same time. Further, it discloses a technique wherein when the temperature of a paper non-feed region does not decrease, as in a case of feeding a small-sized paper sheet, the ratio of power to a coil that heats a side-end portion of the heating roller is decreased while the ratio of power to a coil that heats a central portion of the heating roller is increased.
Jpn. Pat. Appln. KOKAI Publication No. 2001-312178 discloses a technique wherein there is provided a circuit for independently controlling power to each of a plurality of coils, as in the above-described technique, and the frequency of the circuit is varied to alter the ratio of supply powers to the respective coils, thus making uniform the temperature of the heating roller.
Although there is no particular document, the wire for the coil used in an induction heating device is generally affected by a skin effect due to high frequencies. Thus, litz wire (twisted wire), which is composed of a plurality of twisted fine strands, is used for the wire of the coil. This structure is publicly known.
The outside diameter of the litz wire, which has a substantially circular cross section, is determined by the following formula:outside diameter D=1.155×d×√{square root over (N)} (mm)where d: the outside diameter of an elementary strand (mm), and                N: the number of strands.        
Based on this value, a maximum possible number of turns in cross section of the coil is considered in the prior art.
However, according to the drive method of the coil disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2-270293, there is merely a description that the transistor device is switched at 0 V of the AC power supply. In addition, neither a switching time nor a switching timing is described. Thus, there is no method or solution in order to execute a fine temperature control.
Furthermore, if the litz wire is formed with the aforementioned diameter, there is a limit to the density of coil winding in cross section.